High-Tc superconductors: New insights from angle-resolved photoemission

Recent angle-resolved photoemission (ARPES) studies of the high-T _c superconductors by the Argonne group are briefly reviewed. First we discuss sum rules to establish a spectral function interpretation of the data, and the use of ARPES to obtain the momentum distribution. We then apply these ideas to the normal and superconducting state spectra for B₂Sr₂Cu₂O₈. Among the topics discussed are the Fermi surface, polarization selection rules, bilayer splitting, and the superconducting gap.     

Normal and Superconducting Properties of Cuprates in Multielectron Theory

We consider the doping dependence of the normal and superconducting properties of La_2?x Sr _x CuO₄ in the low energy effective model based on the ab initio LDA+GTB calculations. We have found that two quantum phase transitions (QPT) of the Lifshitz type correspond well to the experimental phase diagram. For superconducting state, we have considered both magnetic and phonon mechanisms of pairing. Finally, we compare the true Fermi surface and the spectral intensity map seen in ARPES within a new norm conserving cluster perturbation theory (NC-CPT).     

Multicomponent Order Parameter in the YBa2Cu3O7−δ

We have analyzed the temperature dependencies of the superfluid density in YBa₂Cu₃O₇ along a- and b- crystallographic axes using the multicomponent order parameter for the superconducting gap. Estimated values of the gap components for the d-wave and the isotropic s-wave are Δ _d =29 meV and Δ _s =5 meV, correspondingly. Band structure parameters were taken accordingly ARPES and neutron scattering data.     

Coupling to Phonons in the Migdal–Eliashberg Approach

The relevance of the strong correlations in the high critical temperature superconductors (HTSC) is well experimentally documented. However, if the properties of the normal and superconducting state in HTSC oxides are interpreted in terms of the standard Eliashberg theory, which holds in low temperature superconductor systems, the Migdal–Eliashberg approach implies serious limitations in the reproduction of experimental spectroscopies whose contributions are inter-band and not intra-band. Recent angle-resolved photoemission spectroscopy (ARPES) measurements on HTSC oxides whose contributions are intra-band show a kink in the quasiparticle spectrum at characteristic phonon frequencies in the normal and superconducting state. In contrast with our theoretical discussion, we include our theoretical results for the renormalized energy E _k as a function of the bare band energy ε _k obtained from ARPES in a Pb sample and in a Bi₂Sr₂CaCu₂O_8+δ optimally doped sample (Bi2212). This is clear evidence that electron-phonon is strong and involved in pairing.      

Fermi-surface Shrinking, Interband Coupling and Multiple Gaps in Iron-based Pnictides

In this contribution we present a comprehensive explanation for the origin of the band shifts observed in dHvA and ARPES experiments. Using a four-band Eliashberg analysis, we show that they are a natural consequence of the multiband character of these systems and of the strong particle-hole asymmetry of the bands. We also show that the relative sign of such shifts provides a direct experimental evidence of a dominant interband scattering. A quantitative analysis in LaFePO yields a spin-mediated interband coupling of the order V??0.46?eV, which corresponds to a mass enhancement Z??1.4. We also employ such four-band model to investigate the magnitude of the superconducting gap on different Fermi sheets of Ba_0.6K_0.4Fe₂As₂, and we show that the same four-band model provides a simple explanation of the different gap values on different Fermi sheets and of the thermodynamics properties (specific heat, superfluid density, ??).     

Electronic Spectra of Iron Pnictide Superconductors: Influence of Multi-orbitals Hopping and Hund’s Coupling

We present a theoretical study of the electronic spectral function of iron-pnictide superconductors like LaFeAs(O, F) material. We have attempted two-band tight-binding model Hamiltonian containing various orbitals hopping energies, intra- and inter-band electronic correlations and Hund’s coupling energy in Fe 3d orbitals. The expression of a single particle spectral function within BCS-mean-field Green’s function approach for superconducting state of iron pnictides is obtained. The spectral function at different points of the Brillouin zone is numerically calculated for extended s-wave pairing symmetry as a function of various model parameters applicable for these systems. It is pointed out through numerical analysis that on increasing nearest-neighbor hopping (t ₁) between (d _xz /d _yz ) σ-orbitals, the spectral function A(k,ω) shifts towards the Fermi level and provides favorable conditions for band splitting effects in the form of two well separated peaks in the electronic spectral as observed in iron pnictides angle-resolved photoemission spectroscopic (ARPES) measurements. On increasing t ₂ (nearest-neighbor hopping between d _xz /d _yz π-orbitals), the spectral function shows prominent peak with increase in spectral weight close to the Fermi level at the (π,0) and (0,0) points of the Brillouin zone, while the quasi-particle peak shifts away from the Fermi level at the (π,π) point with decreased spectral weight. The bonding peak of spectral function get suppressed while the antibonding spectral peak becomes prominent with increasing t ₃ (next nearest-neighbor hopping between same orbitals of d _xz /d _yz ) at (π,π) of the Brillouin zone. Further, intra-band Coulomb correlations shift the spectral peak downward with respect to Fermi level along with suppression of the spectral weight. The Hund’s coupling term try to pile up spectral weight close to Fermi level and support to stabilize superconducting state in these systems. The variation of the spectral function within the two-band model is in qualitative accordance with existing ARPES measurements and theoretical investigations in iron-pnictide superconductors.     

Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-T c Cuprates

We argue that the collective mode as observed in angle resolved photoemission spectroscopy (ARPES) on a large class of cuprates can be associated with dynamic incommensurate CDW fluctuations present in these materials. This scenario is substantiated by a comparison of calculated spectra with experimental ARPES data where we obtain a mode frequency which decreases towards optimal doping thus strongly supporting the existence of a quantum critical point around this concentration. Moreover we extract the temperature dependence of the associated bosonic spectrum from ARPES data where it turns out that there is a continuous evolution from mode-type behavior below T _c to a marginal Fermi liquid structure well above T _c.     

Effect of Incommensurate Charge-Density Wave Scattering on the Electronic Structure of High-Tc Cuprates

We argue that the collective mode as observed in angle resolved photoemission spectroslopy (ARPES) on a large class of cuprates can be associated with dynamic incommensurate CDW fluctuations present in these materials. This scenario is substantiated by a comparison of calculated spectra with experimental ARPES data where we obtain a mode frequency that decreases toward optimal doping, thus strongly supporting the existence of a quantum critical point around this concentration. Moreover, we extract the temperature dependence of the associated bosonic spectrum from ARPES data, where it turns out that there is a continuous evolution from mode-type behavior below T _c to a marginal Fermi liquid structure well above T _c.     

Electron Spectroscopy of Single-Layer (n = 1) Bi2Sr2−xLaxCuO6+δ Crystals at Optimal Doping

On single crystals of the single-layer (n = 1) high-T _c superconductor Bi₂Sr_2?x La _x CuO_6+δ at optimal doping (x = 0.4), the electron spectroscopies x-ray absorption (XAS) and high-resolution angle-resolved photoemission (ARPES) were performed. The XAS gives the intensity of the so-called prepeak of the O 1s line what is due to the unoccupied part of the Zhang–Rice (ZR) singlet band. For ARPES, the advantages of single-layer material are the absence of bilayer effects and the possibility to study the electronic properties of the normal state at a sample temperature where the thermal broadening is extremely small (<10 meV). The controlled variation of the polarization vector of the synchrotron radiation made it possible to resolve a distinct fine-structure of the occupied part of the ZR singlet band at the Fermi level. These observations have enormous consequences for line shape analyses and the determination of pseudogaps, and thus the mechanism of high-T _c superconductivity.     

Raman Studies of Anisotropic Magnetic Excitations in Fluctuating Nematic Striped La2−x Sr x CuO4 and the Comparison to Uniform Nd2−x Ce x CuO4

The mechanism of the high temperature hole-doped superconductivity was investigated by Raman scattering. The Raman selection rule is unique, so that anisotropic magnetic excitations in a fluctuating spin-charge stripe can be detected as if it is static. We use different Raman selection rules for two kinds of magnetic Raman scattering processes, two-magnon scattering and high-energy electronic scattering. In order to confirm the stripe effect, the Raman spectra of striped La_2?x Sr _x CuO₄ (LSCO) and nonstriped Nd_2?x Ce _x CuO₄ (NCCO) were compared. The main results in LSCO are (1) magnetic excitations are presented by individual energy dispersions for the k∥ stripe and the k⊥ stripe, (2) the charge transfer is allowed only in the direction perpendicular to the stripe. The direction is the same as the Burgers vector of an edge dislocation. Hence, we assume that a charge moves together with the edge dislocation of the charge stripe. The superconducting coherence length is close to the intercharge stripe distance at x<0.2. Therefore, we propose a model that superconducting pairs are formed in the edge dislocations. The binding energy is related to the stripe formation energy.     

The Nodal SDW Gap and the Superconducting Gap in?BaFe2?x Co x As2

The spin density wave (SDW) transition in BaFe₂As₂ and the superconducting transition in BaFe_1.84Co_0.16As₂ were investigated by Raman scattering. The symmetries of the nodal SDW gap at 400?cm^?1 and the superconducting gap at 75?cm^?1 are both?B _2g. The superconducting coherent peak energy is smaller than the gap energy of the hole pocket, indicating that the peak is the resonant peak in the S _?? superconductor. The superconducting symmetry is given by B _2g in the orbital combination and A _1g (S _??) in the momentum space. The exchange interaction energies are estimated from the two-magnon peak.     

Filtering Properties of Photonic Crystal Dual-Channel Tunable Filter Containing Superconducting Defects

In this work, we analyze filtering properties in a photonic crystal (PC) dual-channel tunable filter. The filter structure containing twin superconducting thin films is denoted as (1/2) ^M S₁(2/1) ^N S₂(1/2) ^M . Here, 1 and 2 are dielectrics of SrTiO₃ and Al₂O₃, respectively. S₁ and S₂ are two high-temperature superconducting thin films taken to be the typical system, YBa₂Cu₃O_7?x . The two channel frequencies can be designed to locate within the photonic band gap (PBG) of the original PC (1/2) ^M . Channel frequencies can be significantly changed by changing N, the stack number of the center PC. With the use of superconducting defects, channel frequencies are temperature-dependent, that is, the filter is thermally tunable. The proposed filter structure is of technical use in superconducting photonic applications at terahertz frequency.     

The Characteristics of the Superconducting and Magnetic Phases in the Polycrystalline Samples of Ruthenocuprates of Nominal Compositions RuSr2GdCu2O8, Ru0.98Sr2GdCu2O8 and Ru0.5Sr2GdCu2.5O8−δ

The temperature dependencies of the resistivity for the superconducting ruthenocuprates of nominal compositions RuSr₂GdCu₂O₈, Ru_0.98Sr₂GdCu₂O₈ and Ru_0.5Sr₂GdCu_2.5O_8?δ were examined for the magnetic field dependent characteristics of the superconducting transitions. The effect of the insignificant diminishing of the Ru/Cu ratio in parent RuSr₂GdCu₂O₈ was confirmed as relevant for the stabilisation of the superconducting phase. Noted differences in the compared characteristics are interpreted for possible inhomogeneous nucleation of the superconducting phase in the parent ruthenocuprate. The phase anisotropy in RuSr₂GdCu₂O₈ and Ru_0.98Sr₂GdCu₂O₈, in presence of the compounds Ru magnetism, appears to be a cause of a significant softening of the H _c2(T) phase line. An anomalous lowering of the magnetoresistivity was observed in the approx. 10 K range above the onset of the superconducting transition, which may suggest the presence of enhanced superconducting fluctuations in the samples. The positive magnetic field shift of the temperatures, which limit the magnetoresistivity and the specific heat signatures of the magnetic ordered state of the Ru sublattice, suggests probing the influence of the ferromagnetic Ru interactions in an effective metallic-like conduction channel present in the samples. Superconducting characteristics of the Ru_0.5Sr₂GdCu_2.5O_8?δ reveal a significant contribution of the Gd paramagnetic signal at low temperatures, interpreted for the presence of a significant anisotropy of the superconducting phase. It is concluded that the Ru–Cu substituted phases of ruthenocuprates may present an opportunity to investigate the effectively anisotropic superconducting phase despite its comparatively high T _c in the compounds related to the 123-type cuprate superconductor.     

BSCCO Superconductors: Hole-Like Fermi Surface and Doping Dependence of the Gap Function

We use the gradient of the energy-integrated angle resolved photoemission (ARPES) intensity in order to define precisely the Fermi surface (FS) in BSCCO superconductors. We show that, independent of the photon energy, the FS is a hole barrel centered at (, ). Then, the superconducting gap along the FS is precisely determined from ARPES measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos(k _x) – cos(k _y) form. A comparison of our ARPES results with available penetration depth data indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.     

Anomalously Weak Cooper Pair-breaking by Exchange Energy in Ferromagnet/Superconductor Bilayers

We report the superconducting transition temperature T _c versus thickness d _F of Ferromagnet/Superconductor (F/S) bilayers, where F is a strong 3d ferromagnet (Ni, Ni _0.81 Fe _0.19 (Permalloy), Co _0.5Fe _0.5) and S = Nb, taken from superfluid density measurements rather than resistivity. By regrouping the many physical parameters that appear in theory, we show that the effective exchange energy is determined from the F film thickness d _F where T _c versus d _F begins to flatten out. Using this rearranged theory, we conclude 1) the effective exchange energy, E _{e x} , is about 15 times smaller than measured by ARPES and five times smaller than deduced in previous studies similar to ours; 2) the dirty-limit coherence length, ξ _F , for Cooper pairs in F is larger than the electron mean free path, ? _F ; and 3) the 3d-F/Nb interface is enough of a barrier that Cooper pairs typically must hit it several times before getting through. The Py/Nb and CoFe/Nb interfaces are more transparent than the Ni/Nb interface.     

Growth of YBa2Cu3O7−x thin films by ion beam sputtering

Polycrystalline, superconducting YBa₂Cu₃O_7−x films have been grown by ion beam sputtering (IBS) of bulk stoichiometric ceramic target. We found that the superconducting properties of the films are very sensitive to the choice of the substrates and post-deposition annealing conditions. Films deposited on SrTiO₃ substrates were found to be the best with a critical onset transition temperature, T_c to be as high as 95 K with a maximum current carrying capacity in superconducting state of 1.5 × 10⁵ A/cm² which is close to the best reported value for single-crystal YBa₂Cu₃O_7−x films.     

High temperature superconducting composite wires

High temperature superconducting composite wires are fast emerging as a new advanced materials technology. Rapid progress has been made towards demonstrating practical levels of superconducting performance across long lengths of high temperature superconducting composite wire fabricated using a manufacturable process. This progress has allowed a first generation of composite wire products that is driving commercialization of high temperature superconductivity. Recent developments have focused on basic materials science and processing advances to significantly broaden the market potential of high temperature superconductivity. These developments have focused on two basic processes, a powder-in-tube approach utilizing Bi₂Sr₂Ca_nCu_n+1O_y (n=1,2) and a coated-conductor approach utilizing Y₁Ba₂Cu₃O_x.     

Study of Hole Pair Condensation based on the SU(2) Slave-Boson Approach to the t-J Hamiltonian; Temperature, Momentum and Doping Dependences of Spectral Functions

Based on the U(1) and SU(2) slave-boson approaches to the t-J Hamil-tonian, we evaluate the one electron spectral functions for the hole doped high T _c cuprates for comparison with the angle resolved photoemission spectroscopy(ARPES) data. We find that the observed quasiparticle peak in the superconducting state is correlated with the hump which exists in the normal state. We find that the spectral weight of the quasiparticle peak increases as doping rate increases, which is consistent with observation. As a conse-quence of the phase fluctuation effects of the spinon and holon pairing order parameters the spectral weight of the predicted peak obtained from the SU(2) theory is found to be smaller than the one predicted from U(1) mean field theory.     

Improvement of Current-Carrying Capacity and In Situ Control of the Superconducting Fraction of MgB2-MgO Composites

Superconducting magnesium diboride-magnesium oxide (MgB ₂-MgO) composite attracts our attention due to its relatively high superconducting transition temperature close to the one of a pure MgB ₂ bulk and metallic transport behavior with low resistivity in the normal state even at a high fraction of insulating MgO. In this paper, we report that the current-carrying capacities of MgB ₂-MgO composites were enhanced by improving the replacement reaction process, and the superconducting MgB ₂ fraction can be in situ controlled by using magnesium (Mg), boron trioxide (B ₂ O ₃), and boron (B) as the raw materials. The composites with mass fractions of superconducting MgB ₂ from 27 to 80 % show that the zero-resistance transition temperatures are above 36.7 K, and the self-field critical current densities are from 0.2 ×10⁶ to 4 ×10⁶ A/cm ² at 10 K. The high J _cS of the superconducting composites under the applied fields prove that there is no appreciable difference between a perfect MgB ₂ sample and one with MgO doping, but the critical current density and flux pinning are improved. These results are meaningful for studying further the flux-pinning mechanism in MgB ₂.     

Anomalous Scattering Rate and Microwave Absorption in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7?δ

We determine the scattering rate from microwave measurements for an optimally doped Bi-2212 single crystal, using a simple two-fluid model with a d-wave symmetry order parameter. In the superconducting state, the calculated scattering rate is three orders of magnitude smaller than that determined from ARPES experiments. A similar anomalously large decrease in the scattering rate is also required to explain the data within a gap-quasiparticle scenario for other HTS, such as YBa₂Cu₃O_7–. The results suggest that the assumption of normal excitations vanishing at low T is invalid and an additional charge mode is responsible for the microwave absorption.